CORROSION PROTECTION OF NDFEB MAGNETS BY SURFACE-COATINGS .2. ELECTROCHEMICAL-BEHAVIOR IN VARIOUS SOLUTIONS

Various surface modification techniques, including single and bimetall ic electroplating, and chemical vapour deposition (CVD), have been app lied to improve the poor corrosion resistance of NdFeB magnets. The co rrosion resistance of the surface-treated samples has been compared wi th that of some commercially available coated samples including nickel , zinc, gold coatings. Potentiokinetic polarisation behaviour in 1N H2 SO4, 10% H3PO4 and Ringer's solution was studied. It was found that in the 1N H2SO4 solution, the corrosion resistance of gold coating was t he best, whereas in chloride-contaminated solution, Ni/Cr and gold coa tings have comparable corrosion resistance. The results of salt spray tests of the coated specimens (in Part 1) were compared with the elect rochemical data. In a simulated marine environment, epoxy coating was found to be slightly better than Ni/Cr coatings. Zn coating provided a certain degree of protection to the magnets in various solutions stud ied. The choice between cathodic, anodic and polymer coatings was foun d to be governed by the service environment and application.